Virus removal by glacier-derived suspended fine sediment in the Arctic

Viruses are a major source of mortality for phytoplankton and bacteria and are therefore seen as drivers of food web dynamics and biogeochemical cycling in the marine pelagic environment. Previous studies have shown that aquatic viruses adsorb to suspended sediment, which theoretically decreases the mortality pressure on their microbial hosts. This process is of particular ecological importance in the Arctic, where coastal systems contain large amounts of suspended fine-sediment, supplied by melting and calving glaciers. The aim of this study was to investigate the effects of glacier-derived fine sediment on marine Arctic microbes during summer in Storfjorden, Svalbard (78 degrees N, 20 degrees E). We sampled for microbial abundances over transects with increasing sediment concentration towards three different glaciers, and examined the adsorption of the natural virus community to previously collected glacier-derived sediment. Our data show declined abundances of phytoplankton (< 20 mu m) and bacteria towards all 3 glaciers. Viral abundances, however, showed an even stronger decline with the virus to bacterium ratio (VBR) reducing from 10-16 in open water to 3-6 in the vicinity of the glaciers. Linear regressions showed negative linear relationships of VBR with turbidity and sediment. This negative relation between suspended sediment and Arctic marine virus abundances is further confirmed by very high adsorption rates of in situ Arctic marine virioplankton upon addition of glacier sediment. Sediment additions (of ecologically relevant concentrations of 100, 200 and 500 mg L-1 to natural seawater) caused viral losses varying between 38 and 66% of the total virus community. Such high viral losses translate into lower contact rates between host and virus, reducing host mortality. Sediment inflow through glaciers may thus affect marine pelagic food web dynamics via viruses, possibly altering the main flow of carbon and other elements in the process. Further study to the possible consequences for food web structure and biogeochemical cycling is essential, as Arctic glacier-derived sediment inflow does not only fluctuate seasonally but is also expected to increase with global warming.